Power plants often employ fuel cell stacks in the production of electricity. These fuel cell stacks comprise a grouping of individual fuel cells, each including an anode, a cathode, and an electrolyte in the form of a matrix disposed therebetween. Also included in the fuel cell stacks are passageways to and from the anode and the cathode to permit the in flow of fuel and oxidant to the anode and cathode respectively, and the out flow of any excess fuel and oxidant, and of byproducts such as water.
Operation of the fuel cell stack to produce electricity includes introducing a fuel such as hydrogen into the anode passageway. Meanwhile, an oxidant such as air is introduced into the cathode passageway. Oxidation of hydrogen occurs at the anode to produce hydrogen ions and free electrons. These electrons flow through an external load, thereby producing electricity, while the hydrogen ions migrate through a matrix to the cathode. At the cathode, the hydrogen ions and free electrons react with the oxygen to form water.
The passageways directing fuel to the anode and oxidant to the cathode are separate and distinct. Since mixing of the fuel and oxidant can result in direct combustion or an explosion, it is imperative to keep these substances separate. As a result, it is important to detect leakage which will allow direct contact between these substances, either over-board leakage or cross-over leakage. Overboard leakage is leakage from the passageways out of the cell which occurs while the fuel and oxidant are being directed to the anode and cathode. cross-over leakage is leakage between the anode and cathode through the matrix within the cells which occurs when a hole develops in the matrix.
Leakage detection has been accomplished using various techniques. One such technique requires monitoring the performance of the cell stack. If performance is low, there may be a failure to direct all of the fuel and/or oxidant to the anode and cathode respectively. Therefore, in order to determine the cause of the poor performance, the operation of the fuel cell stack must be ceased. The individual fuel cells are then examined for leaks using an elaborate, time consuming pressure decay test. This test requires disconnecting the fuel cell, blocking the passageways, filling the fuel cell with inert gas, and utilizing a flow meter to determine if there is any flow and therefore any leaks in the fuel cell. Although this technique detects leakage problems, the problem must be significant in order to be detected and therefore when leaks are detected the situation is critical. This performance monitoring technique fails to detect leakage in its early stages.
Another technique which similarly fails to detect leakage problems sufficiently early to prevent a hazardous situation, is a method of monitoring the sensitivity of the fuel cell stack or individual fuel cells for changes in fuel or oxidant utilization. This technique requires monitoring the performance of the fuel cell stack as excess fuel and then excess oxidant is introduced. Increased performance during either of these introductions signifies leakage. The individual fuel cells can then be monitored to determine exactly which fuel cell is experiencing the leakage by monitoring individual fuel cells for changes in fuel or oxidant utilization. Although this leakage is discovered, the type of leakage remains unknown. This technique can not predict or detect a leakage problem in its early stages before the fuel cell has reached an unsafe condition.
A third technique similarly failing to predict leakage problems is a process of monitoring the fuel cell stack temperature. When cross-over leakage occurs, fuel and oxidant mix together, thereby causing a direct exothermic reaction between the fuel and oxidant. As a result, the temperature of the fuel cell stack increases. Accordingly, a temperature rise in the fuel cell stack or cooling system signifies a cross-over leakage problem. Although this is a simple manner of detecting leakage problems, it is limited by the fact that a significant failure is required in order for a rise in temperature to be detectable. As with the detection techniques discussed above, the fuel cell stack is unsafe by the time the problem is detected.
What is needed in the art is a simple, in-line detection system which monitors leakage, thereby allowing critical conditions to be predicted and shutdown to occur before such a condition has been reached.